Sometimes, call performance specification or control narrative contains the requirements from the user´s viewpoint, including all parameter conditions, and a concrete solution approach. This specification defines WHAT is to be solved and the PURPOSE of the solution and describes the implementation requirements.
The performance specification also defines HOW and WITH WHAT the requirements are to be implemented. Functional description starts with an evaluation of the process flow diagram, including the corresponding process description.
The first thing to do is to put together preliminary control sequences in which only the major control functions are shown; second is to differentiate discrete and continuous control functions – possibly producing other documents, like the sequential function chart (SFC) and the continuous function chart (CFC) – to finally get the functional description (control narrative), which is a text-based description of the control system.
An example:
Flasher level control A flasher inlet consists of a pipe with saturated geothermal water at 160°C with a valve regulated to lower the pressure to 0.6 bar. The water level inside the flasher must be maintained at 1.5 m from the bottom, with a tolerance of 0.5 m. The residual water is taken out with a pump.
To control the level, a variable frequency drive (VFD) is connected to the motor and the discharge is controlled according to the level measured with a transmitter. In case of motor failure or when the level goes above 2.0 m, a control valve opens automatically. This valve should be completely closed below 1.75 m.